Traditionally, the socket portion of a standard total hip replacement consists of a metal hemispheric socket with a porous outer surface to facilitate bone growth into the socket. The socket also includes a liner on the inner surface for articulation with the ball. The femoral side of a hip replacement usually includes a ball supported by a stem component forced into the center, hollow portion of the bone of a patient. The stem is fastened to the femur with either cement or by bone growth into a porous outer surface of the stem. The ball, positioned on the end of the stem, articulates with the liner of the socket and is attached to the stem via a cold weld and trunnion.
A common problem in hip replacements is dislocation. In dislocation, the ball (which is held in the socket by soft tissue around the hip) comes out of the socket causing great pain and the inability to use the hip or walk. The ball must be manipulated back into the socket with sedation or even general anesthesia. The sedation or anesthetic carries risk, as does the manipulation of the hip-components can break or come loose during the manipulation. Once a hip has dislocated, it is more likely to re-dislocate and may happen enough to require repeat surgery. Revision surgery for hip dislocation is often unsuccessful or not worth the risks to the patient.
The most common method of hip dislocation is for the ball to fall out of the bottom of the socket when the hip is flexed, adducted or internally rotated. This dislocation occurs because the ball moves beyond the coverage of the socket with this maneuver and thus slips out of place. Once out of place, it usually stays there and is accompanied by significant spasms and pain.